Francisella tularensis (FT), the causative agent for tularemia, can infect humans by a number of routes, including vector-borne transmission. However, inhalation of the bacterium, and the resulting pneumonic tularemia, is the most dangerous form of disease. This is due to the short incubation time (3-5 days), non-specific symptoms, and a high mortality rate (greater than 80%) in untreated individuals. Furthermore, FT has been weaponized by both the United States and the former Soviet Union making it a viable candidate for use as a biological weapon. Despite over 80 years of research on FT around the world, very little is understood about the dynamic interaction of this bacterium with the host, especially following aerosol infection. Specific Aim 1: Our laboratory has focused on components of the bacterium that are the first encountered by the host following infection, lipids and carbohydrates associated with the outer membrane of the bacteria. Bacterial lipids and carbohydrates are known to be important virulence factors for other pathogens. However, little is known about the role the lipids and carbohydrates play in facilitating infection with FT. Over the past year we have demonstrated that capsule associated with FT directly inhibits the ability of host cells to mount inflammatory responses. We have identified the host pathways targeted by FT capsule to mediate these effects. We have also demonstrated that capsule deficient mutants are not capable of modulating host inflammatory responses or these pathways and that this contributes to their attenuation in vivo and in vitro. Specific Aim 2: There are no vaccines currently licensed for tularemia. Development of novel vaccines has been impaired by the lack of comprehensive understanding both the elements of the bacterium and the host response that are required to drive adaptive immunity against FT. This is, in part, due to a lack of tools that can aid in delineation of protective versus non-protective (as determined by survival) immune responses. Over the past year we made two major advances understanding the requirements for strong adaptive immune responses directed against FT. First, we utilized two strains of the Live Vaccine Strain, which is no longer licensed for use in humans, which engender different degrees of protection against virulent FT. Specifically, one strain (RML LVS) protects all animals against FT while the other (ATCC LVS) does not. Using these strains we demonstrated that presence of effector memory CD4 T cells is strongly correlated with survival against FT challenge among vaccinated mice. Our second advance was the development of an in vitro assay that enables identification of effector cell populations derived from vaccinated animals that are capable of controlling FT replication. We have also generated recombinant strains of LVS and virulent FT that express well defined, but unrelated, CD4 and CD8 T cell epitopes. We are using these strains to follow the antigen specific response among vaccinated animals before and after challenge with virulent FT. This information is revealing differences in the antigen specific response associated with protective versus non-protective vaccination strategies that will be used to develop novel FT vaccines.